In optical communication, an optical semiconductor device such as a semiconductor laser, a semiconductor photodetector, or a semiconductor optical amplifier is used. As the communication amount increases, it is necessary for an optical semiconductor device to operate stably at higher speed.
An optical semiconductor device uses a buried-heterostructure (BH) of a current narrowing structure in which a stripe-shaped mesa unit is buried by using the metal organic vapor phase epitaxy (MOVPE). An optical semiconductor device including the BH can operate stably for a long time.
As a semiconductor laser including the BH, an SI-BH (semi-insulating buried heterostructure) in which a mesa unit including an active layer is buried with a semi-insulating semiconductor layer is known. In a semiconductor laser having the SI-BH, parasitic capacitance of a device as a factor of regulating a modulation bandwidth can be reduced. Consequently, it is advantageous to modulate operation at a high bit rate.
In both a semiconductor photodetector and a semiconductor optical amplifier, using the BH reduces the parasitic capacitance in a device, and realizes higher-speed operation.
From the viewpoint of reduction in the size of an apparatus or simplification of manufacturing process, an optical semiconductor integrated device in which a plurality of optical semiconductor devices are integrated monolithically on the same substrate has been developed.
In such an optical semiconductor integrated device, a device including a mesa structure can be disposed in a predetermined direction on a semiconductor substrate in accordance with the disposing relation with other devices.
Because of cleavableness in the [011] direction or the [0-11] direction, an optical semiconductor device is usually fabricated on the substrate having (100) plane direction.
When the orientation in the longitudinal direction of the mesa structure disposed on the (100) plane has a component of tilt toward the [0-11] direction with respect to the [011] direction, a burying layer which buries the mesa structure grows from both sides of the mesa structure so as to cover the upper side of the mesa structure.
FIG. 1 is a diagram depicting a conventional optical semiconductor device. FIG. 2A is an enlarged cross section taken along line Z1-Z1 of FIG. 1. FIG. 2B is an enlarged cross section taken along line Z2-Z2 of FIG. 1.
An optical semiconductor device 100 includes a semiconductor substrate 105 having the (100) plane, a mesa unit 101 disposed on the semiconductor substrate 105, and a burying layer 104 which buries the mesa unit 101.
The mesa unit 101 includes a core layer 102, a contact layer 103, and a mask 106. The orientation in the longitudinal direction of the mesa unit 101 has a component of tilt toward the [0-11] direction with respect to the [011] direction. Both waveguide end faces in the longitudinal direction of the mesa unit 101 are disposed on the inner side of the cleavage facet of the device, and both waveguide end faces in the longitudinal direction of the mesa unit 101 have the [0-11] direction.
Since the orientation in the longitudinal direction of the mesa unit has the component of tilt toward the [0-11] direction with respect to the [011] direction, the burying layer 104 is formed so as to cover from both sides of the mesa unit 101 to the upper side of the mesa unit 101. The reason is that the burying layer 104 grown along the both side faces of the mesa unit 101 includes a component of growth in a plane tilted from the (011) plane toward the <111>A direction. Consequently, growth in the (111)A plane direction appears above the mesa unit 101, and the burying layer 104 grows so as to cover the mesa like eaves. Particularly, the burying layer 104 is formed so as to cover the upper part of the mesa unit 101 in both end faces in the longitudinal direction of the mesa unit 101 which is the [0-11] direction.
When the top of the mesa unit 101 is covered with the burying layer 104 as described above, it is difficult to form an electrode on the contact layer 103 in the following process.
On the other hand, when the orientation in the longitudinal direction of the mesa unit has only the [011] direction component and the mesa unit extends to the cleavage positions at both ends of the device, the burying layer is not formed so as to cover the mesa unit from both sides of the mesa unit. However, when the orientation in the longitudinal direction of the mesa unit is limited to the [011] direction, design flexibility is limited.
It is therefore proposed to suppress growth of the burying layer over the mesa structure by adding gas containing chlorine to process gas for forming the burying layer.
By adding gas containing chlorine to process gas, even when the orientation in the longitudinal direction of the mesa structure has a component of tilt toward the [0-11] direction with respect to the [011] direction or even when the end faces in the longitudinal direction of the mesa structure are disposed on the inner side of the cleavage position of the device, growth mode of the (111)A plane is suppressed. Consequently, the burying layer which buries the mesa structure is formed without being grown like eaves. Since growth mode of the (100) plane is also suppressed, the burying layer which buries the mesa structure is formed in the same thickness as that of the mesa unit 101 in predetermined distance on both sides of the mesa structure and the thickness decreases after the predetermined distance from the mesa unit 101.    Japanese Laid-open Patent Publication No. 2005-223300    Japanese Laid-open Patent Publication No. 2008-177405    Japanese Laid-open Patent Publication No. 2003-069149    Japanese Laid-open Patent Publication No. 2003-107260